Posterior internal fixation systems undergo internal constraints resulting in high load bearing requirement for the pedicular screw/bone interface. Only few studies deal with the impact of the vertebral augmentation on the migration of pedicular screws. In this study, the impact of the pedicular screw augmentation has been investigated under physiological load for osteoporotic vertebras. The data have been proceeded to reduce the influence of vertebral geometry, which generally leads to results devoid of statistical meaning In 8 osteoporotic vertebrae, two screws have been inserted in each vertebra: a non-augmented on one side and an augmented one on the contralateral side. Compression tests have been performed (two consecutive 50 cycles load steps -100N and 200N-) to observe the displacement of the screw’s head. Two different setups have been employed: a free connection (FC) and a blocked connection (BC). A load step is successful if the migration between two consecutive cycles tends to zero. To reduce the impact of the vertebras’ geometry, the screws’ migration have been compared contra-laterally using the migration ratio (MR). MR of vertebrae is defined as the division of the augmented screw’s migration with the non-augmented screw’s migration. All the augmented screws survived both test setups whereas the non-augmented failed the 200N FC load step. Significant differences are observable only for the highest successful load steps for each test setup: T-tests (P=0.039 and P=0.007 respectively) put into evidence that the results are statistically smaller than one. It is observable as well, that the BC induced fewer loads into the vertebrae: even non-augmented screw can withstand 200N load step. As expected, augmentation of pedicular perforated screws increases their stability in osteoporotic vertebras undergoing large physiological load. This could be explained by the fact that the presence of PMMA increases the load transfer interface improving screw/PMMA complex bearing capacity. Smaller loads induce only small differences that are not significant.
The safety of nucleus implants remains an open issue in the treatment of intervertebral disc degeneration. Post-operative migration and subsequent extrusion represent a high risk of potential unsatisfactory outcome. The effectiveness of additionally sewing a biointegrative nucleus implant into an annulus defect was investigated therefore in this experiment. Laminectomy preserving the facet joints was performed on seven human functional spinal units (FSU’s). A reproducible annulus defect of 6×6 mm was incised, followed by a standard nucleotomy procedure and subsequent introduction of the implants. These woven patches consist of biointegrative, absorbable polyglycolic acid (PGA), lyophilized with hyaluronic acid. The annulus sealing technique requires placing a PGA-patch adjacent to the inner annulus, fixed by sutures (Polysorb 3-0, Syneture) at its four corners. Unsealed annulus defects served as a control group. FSU’s were loaded with a bending torque of 5 to 7.5 Nm. Continual revolution of the specimen around its vertical axis resulted in a combination of lateral, dorsal and flexural bending. During application of loads, implant herniation level was determined every 1 000 cycles according to predefined criteria. Tests were stopped after reaching 20 000 cycles. Five of totally six sewed specimens withstood 20 000 load cycles, whereas only one of five not sewed specimens terminated successfully. Based on the Mann-Whitney test, significant increase in stability can be detected for the sewed procedure. Sewing a biointegrative annulus implant into an annulus defect improves nucleus implant containment. It remains to be shown whether this annulus sealing technique is also effective in highly degenerated annulus tissue. Furthermore, a minimally invasive implantation device is crucial for application in a clinical setting.
